Steps to reproduce the issue
- Go to System > Email Templates > Options
- Set the Mailformat to HTML and save and close
- Create Mail Template for eg. German
- Switch HTML-Body to «yes» and insert a Text, an image and various formats (H1, H2) usw.
I used the TinyMCE as default Editor
My Example looks like this

5. Save the content
Expected result
The content is saved
Actual result
The content is not saved and the Button from the HTML-Body is set back to «No»
System information (as much as possible)
PHP 7.2.x
Joomla 4.0 B1 dev Version Nightly Build from 26.03.
Nginx Webserver (localhost) and reproduced with joomla.com online Version
Additional comments
@Hackwar Any idea?